On the S2 the wipers share the same earth connection as the engine ECUs. I would double check this connection first as you mention a misfire which a poor earth connection at this earth can cause.

At the wiper motor you should have a permanent +12v and a permanent earth. You should also have a pulsed +12v to trigger the wipers when on the intermittent wipe which changes to a permanent +12v when on full speed.

If the wipers fail to park then i would think its either a faulty/poor permanent +12v feed or a faulty/poor earth.